non-autoregressive transformer training scripts error

[ChloeKiwi]

#!/usr/bin/env bash
fairseq-train \
    data-bin/wmt14_en_de_distill \
    --save-dir checkpoints \
    --ddp-backend=legacy_ddp \
    --task translation_lev \
    --criterion nat_loss \
    --arch nonautoregressive_transformer \
    --noise full_mask \
    --share-all-embeddings \
    --optimizer adam --adam-betas '(0.9,0.98)' \
    --lr 0.0005 --lr-scheduler inverse_sqrt \
    --stop-min-lr '1e-09' --warmup-updates 10000 \
    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
    --dropout 0.3 --weight-decay 0.01 \
    --decoder-learned-pos \
    --encoder-learned-pos \
    --pred-length-offset \
    --length-loss-factor 0.1 \
    --apply-bert-init \
    --log-format 'simple' --log-interval 100 \
    --fixed-validation-seed 7 \
    --max-tokens 8000 \
    --save-interval-updates 10000 \
    --max-update 300000

when I use the above bash script,it occurs an error:
fairseq-train: error: argument --ddp-backend: invalid choice: 'legacy_ddp' (choose from 'c10d', 'no_c10d')